In conventional packet, frame or cell based systems there are typically two modes of communication: point-to-point (also known as Unicast) and point-to-multipoint (also known as Multicast). Multicast addresses typically differ from Unicast addresses in that they refer to an intermediate abstraction known as a group or group address. All senders address their transmitted information to this group and all receivers are "tuned" to "listen" to that address to receive the information transmitted to that group by the senders. The senders of information are thus effectively de-coupled from the set of receivers. Senders do not need to know who the receivers are--they simply transmit packets addressed to the group. Similarly, receivers do not need to know who the senders are--they simply send a request to the network (routers) to join a specific group of interest.
Multimedia distribution and conferencing/collaboration systems are advantageously and efficiently supported by Multicast communication methods. As will be used herein, a specific Multicast communication is referred to as a session. In the prior art, it is not possible for Unicast endpoints to access Multicast sessions, due to the differences in addressing modes and receiving modes. This disadvantageously limits the ability of a user at an endpoint client connected to a Unicast network to participate in sessions in which they have interest and in which they could be an active participant.
In co-pending patent application, Ser. No. 08/927,426, filed on Sep. 11, 1997, and assigned to the present assignee hereof, which application is incorporated herein by reference, a method and a system for a endpoint client on a Unicast network to access a Multicast Intenet Protocol (IP) session is described. As described therein, inter-connectivity between a endpoint client connected to a Unicast network and one or more clients connected to a Multicast network is effected through a Multicast-Unicast Server (MUS). Such a server obtains information about sessions on the Multicast network and makes such information available to the client on the Unicast network upon request by the client. Upon being presented with a list describing the subject matter of each session, the user on the Unicast network selects the session to which he or she wants to join, which causes the Multicast-Unicast server to join the appropriate session on behalf of the requesting client for each media type for which the joining client wants to be a participant. The server then sets a bi-directional Unicast User Datagram Protocol (UDP) stream between itself and the client. All packets then received by the server from the client on the Unicast network are address-translated to the appropriate Multicast session address. In addition, all packets received by the server on the Multicast session address are address-translated and sent to the client on the Unicast network. The client on the Unicast network is then able to participate in the Multicast session as both a sender and a receiver of packets to and from other clients which are active during the session.